Fix -Wpointer-bool-conversion warning in FunctionView

LLVM PR 204944 updated Clang's -Wpointer-bool-conversion and
-Wtautological-pointer-compare to look through lvalue references to
functions and warn when they are evaluated in a boolean context or
compared against null.
When passing a function designator/name without '&' to
webrtc::FunctionView, template argument deduction infers the type as an
lvalue reference to a function. Previously, FunctionView used a single
constructor for both function pointers and function references, which
checked 'f ? ... : nullptr'. Evaluating a function reference in this
ternary check triggered -Wpointer-bool-conversion.
This change splits the constructor into two mutually exclusive
constructors:
1. One for function pointers (using std::is_pointer), retaining the
   null check.
2. One for function references (using std::is_function), initializing
   call_ directly without the null check since function references can
   never be null.

Bug: chromium:531796076
Change-Id: Ibb798adddb47c5a110232e4caf956eba2a0743fe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/488020
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Zequan Wu <zequanwu@google.com>
Reviewed-by: Denise Tell <dct@google.com>
Cr-Commit-Position: refs/heads/main@{#48172}